There are now a number of well established techniques for increasing the security of certain types of document. Such “security documents” include banknotes (including paper and plastic currency), bonds, legal documents, identification documents and other documents where the authenticity of the document is extremely important.
Such documents are often provided with one or more overt or covert “security features”, these including specialist inks, optically variable elements, watermarks, security threads, specialist printing techniques and particular substrate materials. These security features are used to authenticate or discriminate between documents either by manual inspection, or more often, by various automatic methods. For example, it is possible to use magnetic techniques to detect the presence of magnetic material in the security threads or printing inks. Certain printing techniques are also used which produce surface relief which can in turn also be detected automatically. Thus in many such automatic methods, various sensors are provided to generate data relating to the particular documents, the data from the security features in particular being used to distinguish between document types and between genuine and counterfeit documents. In many cases, various optical methods are used, in transmissive or reflective arrangements, including infrared and ultraviolet measurements, so as to distinguish between the different types of document in the desired manner.
There is an ongoing need to improve the range of methods by which automatic analysis of security documents may be performed. This not only provides additional performance in terms of the accuracy of distinguishing between document types, but also provides advantages in combating the ever increasing sophistication of counterfeit documents.